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Below are the 2 most recent journal entries recorded in the "Сообщество, посвящённое ра" journal:| September 29th, 2011 | |
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| 07:48 pm [industrialterro] [Link] |
Chitinozoa Chitinozoa (singular: chitinozoan, plural: chitinozoans) are a taxon of flask-shaped, organic walled marine microfossils produced by an as yet unknown animal. Common from the Ordovician to Devonian periods (i.e. the mid-Paleozoic), the millimetre-scale organisms are abundant in almost all types of marine sediment across the globe. This wide distribution, and their rapid pace of evolution, makes them valuable biostratigraphic markers. Their bizarre form has made classification and ecological reconstruction difficult. Since their discovery in 1931, suggestions of protist, plant, and fungal affinities have all been entertained. The organisms have been better understood as improvements in microscopy facilitated the study of their fine structure, and there is mounting evidence to suggest that they represent either the eggs or juvenile stage of a marine animal. The ecology of chitinozoa is also open to speculation; some may have floated in the water column, where others may have attached themselves to other organisms. Most species were particular about their living conditions, and tend to be most common in specific paleoenvironments. Their abundance also varied with the seasons. Chitinozoa range in length from around 50 to 2000 micrometres. They appear dark to almost opaque when viewed under an optical microscope. External ornamentation is often preserved on the surface of the fossils, in the form of hairs, loops or protrusions, which are sometimes as large as the chamber itself. The range and complexity of ornament increased with time, against a backdrop of decreasing organism size. The earliest Ordovician species were large and smooth-walled; by the mid-Ordovician a large and expanding variety of ornament, and of hollow appendages, was evident. While shorter appendages are generally solid, larger protrusions tend to be hollow, with some of the largest displaying a spongy internal structure. However, even hollow appendages leave no mark on the inner wall of the organisms: this may suggest that they were secreted or attached from the outside. There is some debate about the number of layers present in the organisms' walls: up to three layers have been reported, with the internal wall often ornamented; some specimens only appear to display one. The multitude of walls may indeed reflect the construction of the organism, but could be a result of the preservational process. Immature" or juvenile examples of Chitinozoans have not been found; this may suggest that they didn't "grow", that they were moults (unlikely), or that the fossilisable parts of the organism only formed after the developmental process was complete. Most chitinozoans are found as isolated fossils, but chains of multiple tests, joined from aperture to base, have been reported from all genera. Very long chains tend to take the form of a spring. Occasionally, clusters or condensed chains are found, packed in an organic "cocoon". It is not immediately clear what mode of life was occupied by these improbably shaped fossils, and an answer only becomes apparent after following several lines of reasoning. The fossils' restriction to marine sediments can be taken as sound evidence that the organisms dwelt in the Palæozoic seas - which presents three main modes of life:
An infaunal mode of life can be quickly ruled out, as the fossils are sometimes found in alignment with the depositing current; as nothing attached them to the bottom, they must have fallen from the water column. The ornament of the chitinozoans may cast light on the question. Whilst in some cases a defensive role - by making the vessel larger, and thus less digestible by would-be predators - seems probable, it is not impossible that the protrusions may have anchored the organisms to the sea floor. However, their low-density construction makes this unlikely: perhaps more plausible is that they acted to attach to other organisms. Longer spines also make the organisms more buoyant, by decreasing their Rayleigh number (i.e. increasing the relative importance of water's viscosity) — it is therefore possible that at least the long-spined chitinozoans were planktonic "floaters". On the other hand, the walls of some chitinozoans were probably too thick and dense to allow them to float. Whilst little is known about their interactions with other organisms, small holes in the tests of some chitinozoans are evidence that they were hosts to some parasites.[5][9][10] Although some forms have been reinterpreted as "pock-marks" caused by the disintegration of the diagenetic mineral pyrite, the clustering of cylindrical holes around the chamber — where the flesh of the organism was likely to be concentrated — is evidence for a biological cause. Corals in Gotland with daily growth markings have been found in association with abundant chitinozoans, which allow the detection of seasonal variation in chitinozoan abundance. A peak in abundance during the late autumn months is observed, with the maxima for different species occurring on different dates. Such a pattern is also observed in modern-day tropical zooplankton. The diversity of living habits is also reflected by the depth of water and distance from the shore. Different species are found in highest abundance at different depths. While deeper waters around 40 km from the shoreline are generally the optimal environment, some species appear to prefer very shallow water. On the whole, chitinozoans are less abundant in turbulent waters or reef environments, implying an aversion to such regimes when alive, if it is not an effect of sedimentary focusing. Chitinozoans also become rarer in shallower water - although the reverse is not necessarily true. They cannot survive freshwater input.
Tags: Вымершие одноклеточные, Девон |
| August 4th, 2011 | |
| 01:10 pm [industrialterro] [Link] |
Акритархи Acritarchi (греч. acritarcha - неопределенный, неясного происхождения). Акритархи представляют собой искусственную группу неясного систематического положения. Они имеют вид микроскопических капсул шарообразной, эллиптической или дискоидальной формы. Размеры акритарх колеблются от 8-500 мкм до 1 мм.
Репродукция:
Tags: Вымершие одноклеточные, Кембрий |
